1. Field of the Invention
The present invention relates to a plasma addressed display having a flat panel structure in which a display cell is laminated to a plasma cell, and particularly to structures of electrodes and barrier ribs in discharge channels formed in the plasma cell.
2. Description of the Related Art
A plasma addressed display having a plasma cell used for addressing a display cell has been disclosed, for example, in Japanese Patent Laid-open No. Hei 4-265931. As shown in FIG. 3, the plasma addressed display in this document has a flat panel structure including a display cell 1, a plasma cell 2, and a common intermediate substrate 3 interposed therebetween. The plasma cell 2 is composed of a lower substrate 8 joined to the intermediate substrate 3 with a specific gap put therebetween. An ionizable gas is sealed in the gap between the lower substrate 8 and the intermediate substrate 3. Anode electrodes 9A and cathode electrodes 9K, each of which are formed in a stripe shape, are alternately disposed on the inner surface of the lower substrate 8. These electrodes can be printed and sintered on the flat substrate 8 by screen printing, for example. Barrier ribs 10 are formed on the anode electrodes 9A, and they divide the gap, in which the ionizable gas is sealed, into discharge channels 12. Each cathode electrode 9K is positioned between the two adjacent ones of the barrier ribs 10. These barrier ribs 10 can be over-printed by screen printing. The top portions of the barrier ribs 10 are brought in contact with the bottom surface of the intermediate substrate 3. In some cases, the barrier ribs 10 may be formed by sand blasting. Specifically, an insulating material, which is previously formed on the lower substrate 8 to a thickness corresponding to the dimension of the gap, is etched through a mask by sand blasting, to be thus patterned into a stripe shape. The mask can be formed by photolithography. Such a sand blasting process allows the barrier ribs 10 to be patterned at a higher accuracy as compared with the screen printing process or the like. One discharge channel 12 contains one cathode electrode 9K and two anode electrodes 9A positioned on both sides of the cathode electrode 9K. A plasma is discharged between the cathode electrode 9K and the anode electrode 9A. The intermediate substrate 3 is joined to the lower substrate 8 with glass frit 11 or the like.
The display cell 1 is composed of a transparent upper substrate 4. The upper substrate 4 is stuck on the intermediate substrate 3 with a specific gap put therebetween using a sealing material 6 or the like. The gap between the upper substrate 4 and the intermediate substrate 3 is filled with an electro-optic material such as liquid crystal 7. Signal electrodes 5 are formed on the inner surface of the upper substrate 4. These signal electrodes 5 are perpendicular to the stripe shaped discharge channels 12. Pixels are arranged in a matrix at portions where the signal electrodes 5 cross the discharge channels 12.
The plasma addressed display having such a configuration is driven by scanning rows of the discharge channels 12 for performing plasma discharge in such a manner as to switch the discharge channels 12 in line-sequence, and applying image signals to columns of the signal electrodes 5 on the display cell 1 side in synchronization with the scanning of the discharge channels 12. When plasma discharge is generated in each discharge channel 12, the interior of the discharge channel 12 comes to be at a nearly equal anode potential, so that pixels for each row are selected. In other words, the discharge channel 12 functions as a sampling switch. When an image signal is applied to each pixel in such a state that the plasma sampling switch is conductive, sampling is performed. The lighting on or off of the pixels can be thus controlled. Even after the plasma sampling switch is turned into the non-conductive state, the image signals are held in the pixels as they are.
FIG. 4 is a typical perspective view showing structures of electrodes and barrier ribs formed on the lower substrate 8 shown in FIG. 3. The anode electrodes 9A and the cathode electrodes 9K, each of which are patterned in a stripe shape, are alternately arranged. These electrodes can be formed by screen printing or sand blasting. The barrier ribs 10 are formed on the anode electrodes 9A. The width of the barrier rib 10 is narrower than that of the anode electrode 9A. The barrier ribs 10 can be also formed by screen printing or sand blasting.
FIG. 5 is a typical sectional view showing another example of the related art plasma addressed display. Parts corresponding to those shown in FIG. 3 are indicated by the same reference numerals for an easy understanding. In this related art example, a pair of an anode electrode 9A and a cathode electrode 9K are formed in each discharge channel 12. Differently from the related art example shown in FIG. 3, a barrier rib 10 is provided not on the anode electrode 9A but directly on the surface of the lower substrate 8.
In the related art shown in FIG. 3, since the anode electrode 9A is formed on the underside of the barrier rib 10, the width of the anode electrode 9A can be made relatively larger. This brings an advantage that the electric resistance of the anode electrode 9A can be made smaller. Since the cathode electrode 9K is positioned at an approximately central portion of the discharge channel 12, the possible positional offset of the barrier rib 10 does not exert a large adverse effect on the discharge characteristics of the discharge channel 12. This brings an advantage that it is not required to strictly specify the relative positional accuracy between the electrode and the barrier rib. However, since the barrier rib 10 is formed on the anode electrode 9A, there possibly occurs a problem that the bonding strength therebetween is poor. In particular, in the case of forming the barrier ribs 10 by sand blasting, there is a fear that the barrier ribs 10 are peeled from the anode electrodes 9A due to a stress such as a wind pressure applied during the sand blasting step.
On the contrary, in the related art shown in FIG. 5, since the barrier ribs 10 are formed on the surface of the lower substrate 8, the bonding strength is superior to that in the related art example shown in FIG. 3. However, since the cathode electrode 9K is disposed at a position offset from the center of the discharge channel 12, an electric field distribution applied to the cathode electrode 9K tends to be affected by the barrier ribs 10 on both the sides, resulting in uneven plasma discharge. In general, the cathode electrode 9K is easily affected by the electric field distribution as compared with the anode electrode 9A. In the related art example shown in FIG. 5, the cathode electrode 9K is essentially offset from the center of the discharge channel 12, and accordingly, when the barrier rib 10 is offset, there is a tendency to generate uneven plasma discharge. In other words, in the related art example shown in FIG. 5, the discharge characteristics of the discharge channel 12 are largely dependent on positional accuracies of the electrode and barrier rib, which has been a problem with production of the plasma addressed display. Moreover, in the case of AC driving of the plasma addressed display in which the polarity of a signal voltage applied to the liquid crystal 7 is reversed for each row, there is a fear that the signal voltage is not sufficiently applied to the liquid crystal 7 at a position near the barrier rib 10. In the related art example shown in FIG. 5, when back light is impinged from the plasma cell 2 side and an image is observed on the display cell 1 side, there occurs light leakage near the barrier rib 10, causing a problem in reducing the display contrast. In general, the plasma addressed display is observed substantially at a frontal viewing angle. In the related art example shown in FIG. 5, if light coming through gaps between the barrier rib 10, the anode electrode 9A, and the cathode electrode 9K is not sufficiently controlled by the liquid crystal 7, the display contrast is reduced.